A conventional floor structure of a cabin that includes a bed behind a driver's seat is described, for example, in Patent Document 1. The structure in Patent Document 1 includes a floor pan (floor panel) that forms the floor of the cabin. The floor pan includes: a central portion which is located substantially at the center of the vehicle; two side portions which are provided respectively at both sides, in the vehicle-width direction, of the central portion and on which seats for occupants can be installed; and a horizontal portion which is joined to the rear edges of the central portion and of the two side portions and on which a bed for the occupants can be mounted. The floor pan is disposed on a main sill. The main sill includes: parallel portions which are fixed respectively to the bottom surfaces of the two side portions and extend from the front edges of the two side portions to the horizontal portion; and a base portion which is continuously connected to the rear edges of the parallel portions and is fixed to the bottom surface of the horizontal portion of the floor pan so as to extend in the vehicle-width direction.    Patent Document 1: JP-A 10-129531